It is known that the characteristics of certain kinds of dielectric fluids vary when the dielectric fluids are subjected to electric fields. In case of liquid crystals, for example, when a voltage is applied to liquid crystal compounds, orientation properties of the compounds are varied to thereby vary light transmittance of the compounds. It is also known that, when a voltage is applied to heterogeneous fluids containing particles or the like, properties of the fluids such as viscosity are varied by Winslow Effect.
There are, however, problems in the fluids whose properties are varied upon application of a voltage. For example, the liquid crystal compounds are very expensive, or the heterogeneous fluids show poor dispersion stability.
The present inventors have found such a novel effect that some specific fluids move upon application of a voltage, and already applied for patents on the specific fluids (electro-sensitive movable fluids) and micromotors using the electro-sensitive movable fluids (see: Japanese Patent applications No. 16871/1996, No. 16872/1996, No. 76259/1996, No. 248417/1996 and No. 241679/1996) which form the basis of co-pending U.S. Pat. Application Ser. No. 08/792,544, filed Jan. 31, 1997. The micromotors described in these publications show increased output power when they are miniaturized.
In order to more efficiently drive the micromotors disclosed in the above publications, they should be improved for the miniaturization. The motors described in the publications are those of rotor rotation type, and any linear motor which is linearly driven is not described. Further, any pump using the electro-sensitive movable fluid is not described either.
When a voltage is applied to an electro-rheological fluid (ER fluid), its hydrodynamic properties such as viscosity greatly vary reversibly at a high speed correspondingly to the applied voltage. The fluids showing these properties are broadly divided into heterogeneous type (particle dispersion type) and homogeneous type. As the heterogeneous ER fluid, a dispersion obtained by dispersing fine particles such as silica gel in an insulating oil is known.
The heterogeneous ER fluids, however, have a problem in that sedimentation or flotation of particles takes place because of a difference in specific gravity between the particles and the medium. Even if the particles and the medium have the same specific gravity, the same problem of sedimentation or flotation of particles takes place with time because the temperature dependence of the specific gravity of the particles and that of the specific gravity of the medium are different from each other at low or high temperatures. Moreover, the dispersed particles of the heterogeneous ER fluid form a chain structure when a voltage is applied, and therefore, the hydrodynamic properties of the fluid are changed. With the formation of the chain structure, not only increase of viscosity but also development of elasticity takes place, and the fluid exhibits mechanical response approximate to a solid state. For this reason, linear control of the heterogeneous ER fluids is difficult, and in many cases, complicated control means such as feedback control is necessary.
Of the homogeneous ER fluids, a liquid crystal is known as an ER fluid which exhibits no elasticity. The homogeneous ER fluids have ease of controlling because they exhibit no elasticity even when a voltage is applied, and they are free from problems of particle sedimentation and particle flotation because they are homogeneous. However, the homogeneous ER fluids such as liquid crystals are very expensive, so that they are not broadly employed for an industrial use, liquid crystals are only used for, for example, display devices of extremely high value added. Further, the liquid crystals which are the homogeneous ER fluids can be driven only in such a temperature range that the liquid crystals are in the liquid crystal state, so that the temperature range wherein the liquid crystals can be used as the ER fluids is extremely narrow. Though the estimated temperature range wherein the ER fluids are used is from about -30.degree. C. to about 120.degree. C., the liquid crystals cannot be driven in such a wide temperature range.
As described above, the homogeneous ER fluids are advantageous as the ER fluids from the control viewpoint, but they are very expensive and their working temperature range is narrow. On the other hand, the heterogeneous ER fluids are relatively inexpensive, but they are difficult to control and have a problem of fluid stability such as occurrence of particle sedimentation or particle flotation.
In the paper No. 96-252, pp. 437-438, of the 8th symposium on "Dynamics relating to Electromagnetic Force", there is description about "Researches on Electrostatic Devices (New Stress-Transfer System Using Fibers)". The particles dispersed in the heterogeneous ER fluid form a chain structure when a voltage is applied to the heterogeneous ER fluid, as described above. This mechanism is applied to the electrostatic devices of the above paper. That is, instead of the particles, an electrode provided with woven fabric on its surface is used in a silicon oil, and a voltage is applied to the "woven fabric electrode", whereby a chain structure equivalent to the particle chain structure of the heterogeneous ER fluid containing particles is formed by the woven fabric to thereby develop hydrodynamic properties of the ER fluid. In other words, instead of using an ER fluid containing particles, using a silicon oil woven fabric free from sedimentation or flotation is bonded to an electrode material and the woven fabric is used as an electrode. By the application of a voltage, fibers of the woven fabric are allowed to stand up, and dynamic resistance of the upstanding woven fibers is produced to control the fluid. Further, the manuscript collection (pp. 203-206) of the 39th automobile control association lecture meeting (Oct. 16, 17, 18, 1996) discloses "New Torque-Transfer System Using Fibers", and describes that, when fabric is adhesion bonded to a circular plate and the circular plate is rotated in an electric field, the shear stress is increased.
In the above methods, it is explained that the woven fabric has a rigid structure upon application of a voltage and is orientated in the direction of the electric field thereby to increase the shear stress. In the methods, therefore, the woven fibers which are swayed by the fluid when no voltage is applied are made rigid and orientated by applying a voltage, so that the fibers can resist the relative motion of the fluid to the electrodes, whereby increase of shear stress during application of a voltage is accomplished. In the methods, accordingly, only a silicon oil is used, and hydraulic oil constituting a machine part or a working mechanism is never employed, further applicability of the methods to a mechanism.
In the above methods, further, the conductive electrode material is not exposed out at all and is evenly covered with the woven fabric. Moreover, there is no report about production of a jet flow. It is described that the fibers of the woven fabric are swayed in the non-electric field according to the shear rate. Accordingly, flow property control mechanism of the above methods is different from the mechanism of the invention invented by the present inventors, that is, the shear stress is produced by virtue of formation of a jet flow. The shear stress produced in the present invention has hydrodynamic continuity, is free from yield stress which indicates solidification and has ease of controlling, while the above-mentioned fixed electrodes provided with woven fabric do not show these properties.